Mass produced paper and some similar textile products typically are prepared from a stock that is a dilute suspension of fibers in water of a specified low concentration. The concentration of fibers in suspension is often referred to as the "consistency" of the stock. Controlling the consistency of the stock through the pulping, pulp bleaching, and paper making processes is important for maintaining control over these processes. The consistency of the paper stock directly affects the efficiency of each of the processes.
Typically, the consistency of the stock varies from about 3 to 30 percent in the pulping and bleaching processes. Current pulping and bleaching processes are usually accomplished at a consistency of from about 10 to 15 percent. Stock consistency for paper making operations varies from about 0.5 to 5 percent.
Online devices are available for determining the consistency of dilute paper stock at consistencies of about 5 percent or less. Online devices are expensive and typically only one location on the process line is chosen for online determination of stock consistency. Online devices usually do not provide sufficiently accurate results at consistencies above about 5 percent. Laboratory methods are used for spot checking, trouble shooting, and for determination of stock consistency at consistencies above about 5 percent.
One laboratory method in general use is based on the gravimetric principle and is subject to large sampling error. This method is time consuming, prone to human and instrument errors, and examines a relatively small sample of 10 to 30 grams compared to a much larger heterogeneous volume of stock that is actually used in the pulping, pulp bleaching, and paper making processes.
Paper stock typically has the appearance of a thick slurry of fibers in water at consistencies above about 5 percent. Samples usually vary significantly in density and in volume. Previous attempts to use portable nuclear gauges to determine the moisture content of paper stock and other materials having variation in sample volume and density generally have not provided consistent results. The gauges are sensitive to volume changes in materials.
Compensation for measuring uncertainty in the use of portable nuclear gauges typically has been accomplished by attempting to maintain consistent compaction and surface conditions for successive measurements of the sample to reduce variations in density and volume. However, consistent conditions are difficult and time consuming to maintain in practice.